This invention relates to an apparatus and method for displaying regions of lightning activity.
Thunder storms characterized by turbulence and electrical activity (lightning) create great dangers particularly to air travel. It is therefore desirable to locate thunder storm activity as accurately as possible so that thunder storms can be tracked, predicted and avoided. Lightning associated with the mature stages of thunder storms generates electrical signals that propagate through the atmosphere. The detection, recognition, accurate measurement and analysis of these electrical signals provide a basis for storm tracking, avoidance, etc.
Lightning flashes are composed of a series of high current lightning strokes, each stroke being preceeded by a lower current discharge called a leader. The duration of electrical activity associated with a lightning stroke varies but in many instances last as much as a hundred microseconds. The initial rise time of electrical signals associated with a lightning stroke almost never exceeds five microseconds. Following the first peak of the electrical signals of a lightning stroke, lesser signals of sub-microsecond duration but with fast rise times (of five microseconds or less) will occur.
U.S. Pat. No. 4,023,408 discloses a storm mapping system which detects electrical activity caused by weather phenomenon such as lightning strokes. The system is intended to operate on the far field (or radiation field) pattern generated by the lightning stroke. According to the disclosure, the far field pattern is characterized mainly by a low frequency spectrum with maximum amplitude signals occurring between seven and seventy three kilohertz (KHZ). A trio of antenna sensors, an electric field antenna and two crossed magnetic field antennas, are used and each is connected to a tuned receiver on a center frequency of fifty KHz. The crossed loop magnetic field antennas are used to locate the lightning signals in azimuth angle by comparing the relative magnitude of the signals induced in the cross loop sensors to the electric field antenna in a conventional manner. The magnetic field signals are time correlated with the electric field signals before integration. This provides some measure of avoiding unwanted noise like signals. Integration of the correlated signals is formed for 0.5 milliseconds but only after the vector sum of the magnetic field sensor signals is found to exceed a predetermined threshold value. The algebraic sum of the magnetic field sensor signals is amplified and then squared. This signal is used to divide the integrator output signals thereby reducing the magnitude of larger correlated integrated signals below the magnitude of smaller ones. These inverted signals then drive a display such as a CRT display to show larger signals closer to the observation point and smaller signals farther away.
This system has been used on aircraft and appears to work well, but it depends heavily on the magnitude of correlated electric and magnetic field signals to provide a measure of the range of the signal from the observation point of the equipment. Accordingly, the accuracy of range estimates may be affected by the variation in the severity of the thunder storms. Also, some of the detailed characteristics of lightning stroke signals are not utilized to discriminate between interfering signals and true lightning electrical signals.
The Ruhnke U.S. Pat. No. 3,715,660, discloses an apparatus for determining the distance to lightning strokes. It does not measure or calculate the direction of the storm. Like U.S. Pat. No. 4,023,408 it discloses the use of crossed magnetic field sensors and an electric field sensor. Discrimination of lightning signals over background and interfering signals is provided by filtering the output of the antenna elements at one kilohertz. The square root of the outputs of the magnetic field sensors are compared with the absolute value of the electric field element to produce a number which is related to the ratio of the magnetic to electric fields. This ratio is related to range according to FIG. 8 of the subject application. The inventor, Ruhnke, first described this curve in a NOAA Technical Report ERL 195-APCL 16.
As disclosed in the Ruhnke patent, range is calculated based on the .vertline.H/E.vertline. ratio curve of FIG. 8 of the subject application. However, the curve shows that ambiguities in range occur for some .vertline.H/E.vertline. values since .vertline.H/E.vertline. decreases after peaking at about 50 Km. No mention is made of how to resolve the ambiguity. Similarly, the sole discriminate for background noise relies on a one kilohertz filter. The full information contained in the details characterizing lightning strokes are not utilized.
Within a few years of Doctor Ruhnke's effort, Doctor E. Krider and associates built a magnitude direction finder utilizing the initial few microseconds of a lightning stroke which provided accurate directions to the channel basis lightning discharges. Tests on a number of storms at distances of ten to one hundred kilometers indicated angular resolution in the range of one to two degrees. Another important observation by Dr. Krider was that the first few microseconds of a wide band magnetic waveform are due to the radiation field term and the general field equation and that the lightning channel near the ground tends to be straight and vertical which minimizes polarization errors. Dr. Krider's instrument did not actively address ranging.